1. Field of the Invention
This invention relates to a color cathode-ray-tube, and more particularly to a color cathode-ray-tube having a coating film formed over the outer surface of a face plate.
2. Description of the Related Arts
In accordance with the increase of the size of a color cathode-ray-tube (hereinafter simply referred to as CRT) and the improvement of the brightness and focusing performances, a voltage to be applied to a phosphor screen disposed on the inner surface of the face plate, or an applied acceleration voltage of an electron beam, has recently been increased. For instance, a high voltage in the range of 25 to 27 kV has been applied to the phosphor screen of the color CRT having the size of 21-inches. However, in the color CRT having the size of 30-inches or more of recent models, a high voltage in the range no less than 30 to 34 kV is applied to the phosphor screen. With this result, the outer surface of the face plate of the color CRT is charged up when turning the power of a television set on and off. This charged-up outer surface of the face plate easily attracts small dust particles floating in the air, and is tainted by these dust particles. Such taintedness causes the brightness performance of the CRT to be impaired. Also, an electric discharge occurs when a viewer approaches the charged-up face plate, which brings discomfort to the viewer.
FIG. 9 is a graph showing variations in electric potential on the outer surface of the face plate of the CRT. The lateral axis of the graph depicts a time (seconds) counted from when the power is turned on and off, while the longitudinal axis of the graph depicts a surface potential (kV). A curved solid line L denotes variations in electric potential on the surface immediately after the power is turned on. Another curved solid line L1 denotes variations in electric potential on the surface right after the power is turned off. To prevent such a charge-up phenomenon occurring on the outer surface of the face plate of the CRT, there has been recently employed an antistatic type CRT for transferring the charge to earth (ground) by forming a flat and smooth transparent conductive film over the outer surface of the face plate.
FIG. 10 is a side elevation view showing the antistatic type CRT. This CRT 3 comprises a neck portion 6 which incorporates non-illustrated electron guns. The CRT 3 further comprises a deflection yoke 7, a funnel portion 13, a face plate 4 and a high voltage button 5. The deflection yoke 7 is connected to a deflecting power source of the deflection yoke 7 via a lead line 7a. Further, the electron guns are connected to a driving source via a lead line 6a. Furthermore, the high voltage button 5 is connected to a high power voltage source by way of a lead line 5a.
In the CRT 3, the electron beam emitted from the built-in electron guns of the neck portion 6 is deflected by an electromagnetic force exerted external to the CRT by means of the deflection yoke 7. Meanwhile, a high voltage is applied to the phosphor screen disposed on the inner surface of the face plate 4 via the high voltage button 5. This applied high voltage accelerates the electron beam, and the energy produced by the bombardment of this accelerated electron beam excites the phosphor screen to illuminate. As mentioned above, the external surface of the face plate 4 tends to charge up by the influence of the high voltage applied to the phosphor screen disposed on the inner surface of the face plate 4.
As one of the countermeasures to prevent such a charge-up phenomenon, a flat and smooth transparent conductive film 1 is formed over the outer surface of the face plate 4. The transparent conductive film 1 is connected to earth (ground), and the charge-up phenomenon on the outer surface of the face plate is prevented by constantly flowing the charge to ground.
In order to connect the transparent conductive film 1 formed over the outer surface of the face plate 4 and the earth, an implosion preventive metal band 8 wound around the side wall of the face plate 4, is connected to the transparent conductive film 1 by means of a conductive tape 12. This implosion preventive metal band 8 is connected to the earth 10A via an earth line 10 caught on a hook 9.
Broken lines M and M1 in FIG. 9 respectively designate variations in electric potential on the outer surface of the face plate 4 soon after the power of the antistatic type CRT 3 shown in FIG. 10 has been turned on and off. It is to be understood that the transparent conductive film 1 significantly reduces the charge on the outer surface of the face plate 4.
Since the transparent conductive film 1 on the outer surface of the face plate 4 involves a hardness and adhesiveness to some extent, the film is generally formed of a coating film made from silica compounds (SiO.sub.2). According to one method of forming such a coating film 1 made from silica materials, after an alcohol solution of silicon alkoxide including a hydroxyl group and an alkoxyle group as a functional group has been uniformly and smoothly applied onto the outer surface of the face plate 4 by means of a spin coating method, the coating film is subjected to a relatively low temperature baking process of about 100 degrees or less.
Since the coating film 1 formed by the above method has a porous property and comprises a silanol group (.tbd.Si--OH), it is possible to reduce an electrical resistivity on the surface of the face plate 4 by absorbing water from the air. However, if this coating film 1 is baked in a high temperature, the hydroxyl group, or --OH, included in the silanol group disappears and the water absorbed in the pores is lost, whereby the electrical resistivity of the coating film 1 is increased and a desirable electrical conductivity is hard to be obtained on the surface of the face plate 4. Therefore, the coating film 1 must be baked in a low temperature, and consequently the strength of the film becomes rather weak. Moreover, if the coating film 1 has been used for a long period under the aired condition, water retained in the porous coating film 1 evaporates, and the electrical resistivity increases with time. Once water has been vaporized away from the porous coating film 1, the coating film 1 cannot absorb water again.
To overcome such a drawback as set forth in the above description, attempts are now being made such as that which gives an electric conductivity to the coating film 1 by combining metallic atoms, e.g. a zirconium (Zr), with the alkoxide structure, but any substantial improvement has not yet been achieved.
As another method of improving the conductivity of the coating film 1, particles of a tin oxide (SnO.sub.2) and an indium oxide (In.sub.2 O.sub.3) are mixed and dispersed, as a conductive filler, into the alcohol solution of silicon alkoxide, and a paint added with a fairly small amount of phosphorus (P) or an antimony (Sb) is uniformly and smoothly applied over the outer surface of the face plate 4 by the spin coating method. Further the face plate coated with the paint is baked at a relatively high temperature of 100 to 200 degrees, for example. In accordance with this method, the strength of the coating film is improved, and it becomes possible to obtain a flat and smooth transparent conductive film 1, the electric resistivity of which is not varied with time under any circumstances.
In recent years, with a strong demand of a high quality color CRT, the improvement of the contrast and the color tone of the luminescence of the color CRT has been put into a practical use by coloring the transparent conductive film on the face plate. Namely, the mixture of a single dye or pigment made from organic or inorganic materials into a paint for producing the transparent conductive film over the face plate enables a colored paint to be obtained. By applying this colored paint onto the outer surface of the face plate and baking this painted face plate by means of the spin coating method, there is obtained a color CRT, shown in FIG. 11, having a coating film with a filter function for selectively absorbing a light within a predetermined range of wavelength, as well as the antistatic function. Specifically, although the color CRT of FIG. 11 appears similar to the color CRT 3 of FIG. 10, the coating film 2 formed over the face plate 4 of the color CRT 11 of FIG. 11 has the optical function and the electrical function as well.
FIG. 12 is a graph explaining the optical characteristic of the electrical and optical coating film 2 in the prior art. In the graph, a lateral axis denotes a wavelength of the light (nm), whereas the vertical axis denotes a relative luminous intensity (%) and a spectral transmittance (%). A curved line B shows a spectral distribution of the relative luminous intensity of blue luminescence on the phosphor screen of the color CRT, and the main spectrum wavelength is about 450 nm. Likewise, curved lines G and R respectively show the relative luminous intensity of the green luminescence and the red luminescence, and their main spectrum wavelengths are about 535 nm and 625 nm, respectively.
Curved lines II and III represent a spectral transmittance distribution of the face plate 4 itself used in the color CRT. The curved line II represents a transmittance distribution of a clear type face plate having a spectral transmittance of about 85% in a visible light region. In the meantime, the curved line III represents a distribution of a spectral transmittance of a tint type face plate having a transmittance of about 50% in the visible light region. It will be evident from the relationship among the spectral distributions of the curved lines B, G and R which represent the relative luminous intensity of the phosphor screen that the less the transmittance of the face plate, the worse the brightness performance of the color CRT is deteriorated. The tint type face plate, however, can effectively eliminate an external light incident on the phosphor screen of the color CRT. This type of the face plate is preferable for enhancing the contrast performance. Consequently, in accordance with the recent tendency in which a stress is laid on a picture quality of the color television receiver, the tint type face plate is widely adopted.
The curved line I represents one specific example of the spectral transmittance distribution of the electrical and optical coating film 2 in the prior art formed over the outer surface of the face plate 4 for enhancing the contrast performance. If an absorption peak point A of the coating film 2 comes close to one of the main spectrum wavelengths in between the main spectrum wavelength of 535 nm of the curved line G and the main spectrum wavelength of 625 nm of the curved line R, the brightness performance of the color CRT will be impaired. Therefore, the peak point A of the absorption band is usually set within the range of about 570 nm, through 610 nm taking into consideration a half band width of the absorption band. Since the light having a wavelength within this range is coincident with a relatively high area of a spectral luminous efficacy of human eyes, a light element of the external light (white light) having the wavelength within this range should preferably be absorbed to be eliminated in the light of the contrast performance. Consequently, it is extremely important to select a dye or a pigment made from organic or inorganic materials having the above-mentioned light absorbing characteristic, and the curved line I indicates a specific example of a pigment or a dye having the absorption peak point A at the wavelength of 572 nm.
Further, in the color CRT 11 having the electrical and optical coating film 2, since the light absorbing characteristic of a dye or a pigment consisting of organic or inorganic materials to be mixed in the coating film has a relatively broad band width, a tail region on the longer wavelength side of the main spectrum wavelength of the green luminescence and a sub peak portion on the shorter wavelength side of the main spectrum wavelength of the red luminescence are absorbed by this coating film. In short, it is possible to improve the color tone of the luminescence of the color CRT 11.
On this point, however, it is difficult to realize the aforesaid light absorbing spectrum of the coating film 2 within the specified range between 570 nm and 610 nm. This is because the pigment or dye consisting of a single organic or inorganic material which satisfies the above mentioned requirement is extremely rare to obtain. As another reason is that even if the light absorption peak itself of the dye or pigment is in the above specified range, other optical characteristics such as the skirt region of the absorption peak and the sub peak point, for example, may not match for the requirement to realize a desired the spectrum absorption in many cases. For these reasons, the appropriate dye or pigment is hard to select.
Furthermore there has been a drawback in the conventional color CRT having an antistatic type selective light absorbing film, since the absorption peak point of the main absorption band is in between 570 nm and 610 nm as the optical characteristic of the antistatic type selective light absorbing film, if the external light (white light) is reflected from the phosphor screen after having been incident on the phosphor screen of the color CRT with the antistatic selective light absorbing film, a large magnitude of lights having the wavelength within this range are particularly eliminated by the main absorption band. Thereby, the reflected light is colored. This drawback will be particularly described hereunder upon reference to FIG. 13. FIG. 13 also shows a spectrum locus (IV) of a blackbody radiation in a CIE standard chromaticity diagram. The points of the locus on a horse shoe shaped diagram shown in FIG. 13 depict the chromaticity point of each single color luminescence. The external light may slightly differ dependent on its type, but is chiefly a collective light flux composed of a plurality of single luminescences, like sun light. Most representative external light has a color temperature of 4500K or thereabout as designated by a point D. The phosphor screen of the conventional color CRT incorporating a face plate without the light absorbing film has achromatic color, or gray. With this phosphor screen, the light absorption is evenly effected across all of the wavelengths of the visible light. The outgoing light reflected from the phosphor screen looks like a natural light having a wavelength component similar to that of the incident light.
Meanwhile, in the case of the conventional color CRT with the selective light absorbing film having the absorption peak point A of the main absorption band at the wavelength of 572 nm as shown in FIG. 12-I, the light having the wavelength of 572 nm or thereabout of the external light (white light) incident on the phosphor screen is absorbed in this main absorption band to be removed. The chromaticity point of the reflected light shifts in the same direction as the direction to which the chromaticity point of the incident light is shifted away from the chromaticity point D of the original external light (white light). Shortly, a vector "a" arises along a line segment connecting between the chromaticity point D of the external light (white light) of 4500K and the chromaticity point of 572 nm of a single luminescence in a direction in which the vector moves away from the chromaticity point of 572 nm of the single luminescence in the chromaticity diagram, and the chromaticity point of the reflected light is shifted. This causes the reflected light to be colored.
In practice, in the case of the color CRT, it is considered that the audience views an original color of the phosphor screen itself when an image is being produced at a black level. Further if the light reflected from the phosphor screen is colored, a black color displayed on the screen looks unnatural. Thus the picture quality of the color television has been impaired to a large extent.
To overcome this drawback in the prior art, an object of the present invention is to provide a color cathode ray tube having an electrical and optical coating film whose absorption peak of an absorption band, superior in optical characteristic, is set within a specified range of wavelength.
Another object of the present invention is to provide a color cathode ray tube having a light selecting film by which a reflected light is not colored even though the absorption peak of the main absorption band is within the range and which is highly effective for improving the contrast performance, between 570 nm and 610 nm.